From mboxrd@z Thu Jan 1 00:00:00 1970 From: Patrick McHardy Subject: Re: bad networking related lag in v2.6.22-rc2 Date: Wed, 23 May 2007 19:16:30 +0200 Message-ID: <465476EE.3090103@trash.net> References: <20070517174533.GA538@elte.hu> <200705221147.56571.kernel@prachanda.hub> <20070522062233.GA20002@elte.hu> <200705231110.44526.kernel@prachanda.hub> <20070523063052.GB26814@elte.hu> <46541DC4.4090501@trash.net> <20070523112559.GA24604@gondor.apana.org.au> <4654269C.4040809@trash.net> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: 7bit Cc: Herbert Xu , Ingo Molnar , Anant Nitya , linux-kernel@vger.kernel.org, Andrew Morton , Thomas Gleixner , "David S. Miller" , Linux Netdev List To: Linus Torvalds Return-path: Received: from stinky.trash.net ([213.144.137.162]:46465 "EHLO stinky.trash.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S935175AbXEWRTg (ORCPT ); Wed, 23 May 2007 13:19:36 -0400 In-Reply-To: Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org Linus Torvalds wrote: > There appear to be other obvious problems in the recent "cleanups" in this > area.. > > Look at > > psched_tdiff_bounded(psched_time_t tv1, psched_time_t tv2, psched_time_t bound) > { > return min(tv1 - tv2, bound); > } > > and compare it to the previous code: > > #define PSCHED_TDIFF_SAFE(tv1, tv2, bound) \ > min_t(long long, (tv1) - (tv2), bound) > > and ponder how that "trivial cleanup" totally broke the thing. > > Hint: "psched_time_t" is an "u64". What does that mean for > > min(tv1 - tv2, bound); > > again, when "tv2" is larger than tv1. It _used_ to return a negative > value. Now it returns a positive "bound" upper bound, because "tv1-tv2" > will be used as a huge unsigned (and thus _positive_) integer. And was > that accidental, or done on purpose? > > Sounds accidental to me, since you then want to return a "psched_tdiff_t", > which is typedeffed to be "long". > > Doesn't sound very safe to me, especially since the commit message for > this is "[NET_SCHED]: turn PSCHED_TDIFF_SAFE into inline function", and > there's no indication that anybody realized that it changed semantics in > the process. I did realize it, but tv2 > tv1 can't happen and makes no sense for the users of this function. I probably should have provided a more detailed changelog entry. > Hmm? What _should_ that thing do? It is used to calculate the amount of tokens a tocken bucket has accumulated since the last refill, thus we always have tv1 >= tv2 (modulo ktime wraps). In fact tv2 > tv1 was never properly supported. This macro would have returned the negative long long value, but all users assign it to a psched_tdiff_t (long), so depending on the exact values, it might still be interpreted as a large positive value. Additionally there was a second implementation for the gettimeofday clocksource that didn't return the negative difference but the bound value.